Electric induction motors are suitable for various applications because of their durable construction. The rotor of an induction motor is not wired directly to any current supply. Instead, a secondary current is induced in the rotor windings by a rotating magnetic field generated by the stator windings. Because the rotor windings are energized by a magnetic field, electric induction motors do not require any brushes to connect the rotor to a power source. The absence of brushes is desirable because brushes for electric motors can wear out and/or require maintenance.
Some electric induction motors have start windings on the stator that are energized along with the main stator windings during motor startup to help get the rotor up to operating speed. Once the motor is at speed, the start windings are de-energized because the motor operates more efficiently at full speed using only the main windings. This is commonly accomplished with an internal switch actuated by a centrifugal actuator. As generally known to those skilled in the art, a centrifugal actuator is connected to the rotor so it rotates with the rotor. After the rotor reaches a threshold speed, centrifugal forces cause weights in the actuator to move in a manner that produces axial movement of the actuator. The movement of the actuator results in movement of a switch arm to disconnect the start windings from the power supply. The switch is typically mounted close to the rotor to facilitate actuation of the switch by the centrifugal actuator on the rotor. The combination of switch and centrifugal actuator is sometimes referred to as a centrifugal switch.
The proximity of the switch to the rotor and other components creates the potential for electrical arcing between the switch and other components of the motor. For example, the switch can be mounted on the end shield of the motor close to a bearing hub for the output shaft of the rotor assembly presenting a potential for electrical arcing between the switch and the shaft or bearing hub. Electrical arcing is undesirable because it can cause structural and electrical damage to the components of the motor. Electrical arcing can also interrupt operation of the motor by blowing fuses and/or tripping circuit breakers. Thus, it is common to provide electrical insulation between the switch contacts and other components of the motor to limit or prevent arcing. For example, after the switch is mounted during assembly of the motor, fish paper or another suitable insulating material can be placed between the switch contacts and other parts of the motor to form a barrier to limit the potential for arcing. Other options used in prior art motors are to enclose the switch in an electrically insulating box or enclose the switch contacts in a boot made of an insulating material. There have also been designs in which plastic barriers are provided on the terminal board to limit arcing. Each of these options has added costs. Further, it is possible that the insulation provided to limit arcing associated with the switch may become dislodged or may be inadvertently omitted during assembly. This would not be apparent because the motor would continue to operate normally, at least until there was an unexpected arcing incident.